Oh, and to answer your question, you may not be able to depending on the personality of your rescue bird and of the birds in your flock. I have raised Cornish-Rock to adulthood (it's ridiculously easy so long as they are forced to mostly free range) and found that they are extremely docile birds. Roosters discover pretty quickly that a Cornish-Rock hen isn't fast enough to outrun him (which, in the chicken world, means that the Cornish hen _really_ digs him - so much so that she doesn't even want to play hard to get). Regular hens will find out that when they're in a bad mood, Cornish hens are too slow to get out of the way of being pecked at. They become the barnyard punching bag.
When I've had them in the past I've only kept them together while free ranging - never in an enclosed space where the Cornish can't escape (and can't hide - anything that rotund bottom will fit through will also fit any other hen you have). When they reach adulthood I'll keep them in a separate pen entirely. Usually until I get tired of feeding them and sell them off to make better use of the space.
Please remember this is just my experience with adult Cornish-Rocks. Some birds are fine in a mixed flock and some mixed flocks will happily accept a Cornish-Rock. But when it comes down to it, these Birdzillas _are_ different and honestly look like they live a happier life when allowed to do nothing but eat and poop for a few months rather than be on a regular diet for years (can you blame them? Who wants to be on a never-ending diet when all you want is FOOD?).
